1. Field of the Invention
The present invention relates generally to an active suspension system which varies suspension characteristics according to vehicle driving condition for achieving high level riding comfort and vehicle driving stability. More specifically, the invention relates to a hydraulic circuit for an active suspension system, which can provide wider adjusting range of suspension characteristics in order to permit softening of suspension characteristics without degradation of attitude change suppressing ability.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European patents 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
In one of the typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. The fluid pump is generally rated to produce rated pressure which is selected in view of the required line pressure in a supply line for supplying the pressurized fluid to the working chamber, at the minimum revolution speed of the engine so that the working fluid pressure to be supplied to the working chamber of the hydraulic cylinder can be satisfactorily high at any engine driving range. As will be appreciated, the output pressure of the fluid pump increases according increasing of the engine revolution speed. Therefore, at high engine revolution speed range, excessive pressure in excess of a predetermined maximum line pressure is relieved via a relief valve. Therefore, the engine output can be wasted to degrade engine driving performance as a power plant for the automotive vehicle and thus degrade fuel economy.
On the other hand, in the practical operation of the active suspension system, the fluid pressure in the working chamber in the hydraulic cylinder can be maintained at constant value for maintaining a desired vehicular height, at substantially low vehicle speed range or while the vehicle is not running. Despite this fact, the prior proposed hydraulic circuits for the actively controlled suspension systems supply the rated pressure of the fluid pump which should be higher than a minimum line pressure required for adjustment of the fluid pressure in the working chamber. In order to maintain the rated pressure to be output from the fluid pump, substantial engine output will be consumed even at the low vehicle speed range, in which the line pressure is not required for no possibility of adjustment of the suspension characteristics.
Improvement in the hydraulic circuit for the prior proposed active suspension system has been proposed in the co-pending U.S. patent application Ser. No. 331,602, filed on Mar. 31, 1989, U.S. Pat. No. 4,911,468 which application has been commonly assigned to the common assignee to the present invention. The corresponding invention to the above-identified co-pending U.S. Patent Application has been published as Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-249509, published on Oct. 4, 1989. The prior invention has been directed to a hydraulic circuit for an actively controlled suspension system which employs first and second pressure relief valves disposed in a hydraulic pressure source circuit for relieving excessive pressure. The second pressure relief valve is provided a lower relief pressure than that of the first pressure relief valve. Means for selectively connecting and disconnecting the second pressure relief valve is disposed in the hydraulic pressure source circuit at an orientation upstream of the second pressure relief valve. The means is positioned at shut-off position to disconnect the second pressure relief valve when a vehicle traveling speed is higher than a predetermined speed. The means is responsive to the vehicle speed lower than the predetermined speed for establishing connection between a pressurized fluid source to the second relief valve for relieving the pressure at lower level than that established when the vehicle speed is higher than the predetermined speed.
In the prior proposed suspension systems as set forth above, active suspension control is taken place for adjusting suspension characteristics by introducing and draining line pressure in a hydraulic circuit. In prior proposed constructions of the active suspension systems, line pressure in a supply line in the hydraulic circuit tends to drop below a set pressure during vehicular bouncing action due to rapid increase of supply amount of the pressurized fluid into the working chamber. Lowering of line pressure may cause lowering damping characteristics in a hydraulic cylinder. In order to avoid this, it is the prior proposed technologies to provide a fixed path area orifice for restricting fluid flow. Typically, such fixed flow path area orifice is provided in a drain circuit for restricting fluid flow in the drain line and whereby restricting magnitude of lowering of the fluid pressure in the working chamber so that sufficient damping characteristics can be maintained even during vehicular bouncing action.
Such prior proposed strategy is effective in terms of providing sufficient damping characteristics against bouncing action of the vehicle. However, on the other hand, the presence of the fixed path area orifice clearly degrade drainage performance of the hydraulic circuit and thus serves to prevent the working fluid in the working chamber to be effectively drained. This causes limitation of softening of suspension systems.